Re: [PATCHv0 dont apply] RFC: kvm eoi PV using shared memory

Avi Kivity
2012-04-10T16:08:28Z

On 04/10/2012 06:14 PM, Michael S. Tsirkin wrote:
> On Tue, Apr 10, 2012 at 06:00:51PM +0300, Avi Kivity wrote:
> > On 04/10/2012 05:53 PM, Michael S. Tsirkin wrote:
> > > > >
> > > > > Yes. But we can and it's easier than figuring out priorities.
> > > > > I am guessing such collisions are rare, right?
> > > >
> > > > It's pretty easy, if there is something in IRR but
> > > > kvm_lapic_has_interrupt() returns -1, then we need to disable eoi avoidance.
> > >
> > > I only see kvm_apic_has_interrupt - is this what you mean?
> >
> > Yes, sorry.
> >
> > It's not clear whether to do the check in kvm_apic_has_interrupt() or
> > kvm_apic_get_interrupt() - the latter is called only after interrupts
> > are enabled, so it looks like a better place (EOIs while interrupts are
> > disabled have no effect). But need to make sure those functions are
> > actually called, since they're protected by KVM_REQ_EVENT.
>
> Sorry not sure what you mean by "make sure" - read the code carefully?

Yes. And I mean, get called at the right time.

> >
> > Better to keep everything per-cpu. The code is in virt/kvm/ioapic.c
>
> Hmm. Disabling for level handles the ack notifiers
> issue as well, which I forgot about.
> It's a tough call. You think looking at
> TMR in kvm_get_apic_interrupt is safe?

Yes, it's read only from the guest point of view IIRC.

> > >
> > > > Why do we care about
> > > > level-triggered interrupts? Everything uses MSI or edge-triggered
> > > > IOAPIC interrupts these days.
> > >
> > > Well lots of emulated devices don't yet.
> > > They probably should but it's nice to be able to
> > > test with e.g. e1000 emulation not just virtio.
> >
> >
> > e1000 doesn't support msi?
>
> qemu emulation doesn't.
>

Can be changed if someone's really interested. But really, avoiding
EOIs for e1000 won't help it much.

--
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